Flat flexible cable is finding ever increasing use in the telecommunications industry, and in such applications it is important that such cable exhibit extremely high crosstalk rejection, be of minimum thickness, and be capable of being folded back upon itself, so as to be applicable for telephone under-carpet cable, applications, one such type being known as TUCC.RTM. flat cable (a registered trademark of the Western Electric Company).
Flat cable, particularly of the 25 pair type designed for telephone system wiring, has taken a number of different forms heretofore, particularly in order to reduce crosstalk to tolerable levels. One of the earliest forms of such cable was comprised of a woven ribbon of twisted pair conductors. Disadvantageously, such cable was relatively thick (on the order of 0.080 inch), and when folded back upon itself so as to allow the cable to change direction when laid under carpet, for example, produced intolerable bumps that were found objectional from even an esthetic standpoint.
Another approach taken to reduce troublesome crosstalk in flat cable heretofore has involved forming socalled pseudo-twists (or crossovers) at predetermined spaced intervals therealong. In the latter type of cable, the conductors are normally initially fabricated along a common plane, with the zig-zag patterns of the respective conductors being spaced apart and arranged relative to each other in distinct groups so that when the fabricated cable is folded back upon itself in the longitudinal direction, the then respectively associated pairs of overlying-underlying conductors in the two groups effectively cross-over each other in a pseudo-twist manner. Unfortunately, when such a once folded cable is again folded back upon itself to effect bends therealong, it to exhibits undesirable thickness, particularly in the areas of the bends when required in under-carpet applications. Moreover, it has also been found quite difficult to manufacture and latter type of flat cable with crosstalk losses in excess of 100 db, particularly in end use lengths greater than 10 to 15 feet.
The criticality of crosstalk in telephone system wiring may perhaps be most readily appreciated when it is realized that the human ear can detect and interpret voice information at signal levels of nearly 100 db-five orders of magnitude down from, or only 0.001 percent of, the desired voice signal. In contradistinction, the crosstalk rejection requirements for flat cable used in computers, as well as in other digital systems, is far less demanding. For example, in such applications, the digital hardware can typically ignore up to one percent crosstalk (20 db loss). It is thus seen that flat cable when used in telephone voice applications must be 100 times more effective with respect to signal crosstalk rejection than when used in digital transmission applications.
Other attempts to reduce crosstalk in flat cable, particularly when the conductors are in overlying-underlying arrays, have involved the use of metallic shields, specially constructed and positioned screens, that separate groups or pairs of conductors, and ground planes. The purpose of such auxiliary members, of course, is to establish desirable capacitive-decoupling electric and magnetic field boundaries about either pairs or groups of conductors within the cable. While such flat cables generally exhibit excellent crosstalk rejection characteristics, they disadvantageously are likewise relatively thick, often are quite inflexible, and are costly to manufacture.
Still another approach taken to reduce crosstalk heretofore has been to vary the spacing between either adjacent conductors forming a given pair, or the spacings between pairs, while lying in a common plane. In many telephone wiring applications, where 25 conductor pairs are generally desired, such cables must necessarily be considerably wider than when the conductors are arranged in two overlying/underlying arrays, with only 25 conductors in each array.
As a result of the aforementioned crosstalk, dimensional and cost problems involved in the manufacture of flat cable for telephone applications heretofore, a completely different approach to attacking these problems evolved in the design of a cable wherein two pecularily aligned, and precisely offset, overlying/underlying arrays of conductors are fabricated in a common insulative medium, such as in an extruded plastic carrier or, in separate flat cables which are thereafter bonded together or, more preferably, are interleaved and laminated between three plastic films.
Considered more specifically, it has been found in such a cable that capacitive decoupling, which gives rise to a very significant reduction in cross talk, can be substantially completely effected provided adjacent conductor pairs therein are spaced and spatially arranged to satisfy the following so-called geometric decoupling equality: EQU d.sub.13 d.sub.24 =d.sub.14 d.sub.23 ( 1)
where the subscripts 1 and 2 represent the conductors of the first conductor pair, and the subscripts 3 and 4 represent the conductors of the second pair.
While it can be shown mathematically that the standard "quad" and "Tee" configurations most ideally satisfy the above decoupling equality, with certain assumptions, other quadrilateral geometric configurations, such as the parallelogram, equilateral trapezoid, and general quadrilateral, can all be shown to at least substantially satisfy the above decoupling equality. However, only the parallelogram configuration is of primary concern herein, because it advantageously allows two overlying/underlying arrays of conductors to be much more closely spaced than in the case of a true "quad", or square geometry. In addition, a parallelogram configuration of adjacent conductor pairs advantageously exhibits an "open" decoupled characteristic, i.e., the decoupling locus thereof, which can be readily derived algebraically, is such that any number of adjacent decoupled conductor pairs can be employed in a multi-pair, multi-arrayed flat cable.
For purposes of this invention, it will suffice to simply state that in a flat cable of the type in question, it has been found that crosstalk is most effectively minimized when a family of parallelograms are chosen that not only satisfy the aforementioned decoupling equality (d.sub.13 d.sub.24 =d.sub.14 d.sub.23), but further satisfy the following decoupling locus equation therefor:
X=(2 cos 2.alpha.).sup.1/2, (2)
where .alpha. is the skew angle, i.e., the acute angle between adjacent sides of the parallelogram, and X=2b/a, where 2b is the length of each side of one pair of parallel sides and a is the length of each side of the other pair of parallel sides. The angle .alpha. approaches .pi./4 as the vertical spacing between conductors becomes very close, or conversely, as the pair spacing increases.
It can be shown from a calculated plot of Near End Crosstalk (NEXT) versus conductor offset, that for a skew angle .alpha. close to .pi./4, and a vertical spacing between the conductors in each pair of approximately 0.004 inch, for example, that the expected loss peak will occur for an offset approximately equal to 0.012 inch. The particular offset required in any given cable of the type in question will depend, of course, on not only the thickness and dielectric constant of the material chosen to separate the upper and lower arrays of conductors, but on the cross-sectional configuration of the latter. As such, it is appreciated that in actual cable manufacture, less then perfect dielectric homogeneties and boundaries will selectively negate the realization of absolute zero crosstalk in any practical flat cable embodiment. Nevertheless, a flat cable of the type of concern herein is capable of producing crosstalk loss in excess of 120 db for cable lengths of 35 feet or more.
For a more detailed theoretical discussion of such a unique cable, reference is made to an article entitled "Geometrically Decoupled Balanced Pairs", by Mr. D. P. Woodard, of Bell Telephone Laboratories, presented at the proceedings of the Internal Wire and Cable Symposium, November 1978.
In connection with the manufacture of one particular form of the last mentioned type of cable for use in telephone under-carpet applications, wherein 25 pairs of conductors are arranged such that adjacent pairs form a parallelogram configuration, and wherein the upper and lower conductors are laminated between a common double adhesive-backed center film, and respectively associated adhesive-backed outer films, a very serious problem has arisen in maintaining not only the spacing between conductors in each array, but the very critical offset between the respectively associated conductors in the two arrays, within stringent limits.
More specifically, it was found that the requisite offset between the two arrays of conductors tended to cause the respectively associated pairs of overlying/underlying conductors to randomly shift laterally while the conductors and films were fed through a pair of laminating rollers during the fabrication of the cable. Unfortunately, it was further found that even extremely small lateral displacements of the conductors, particularly with respect to the offset dimension, could not be tolerated in a cable designed for telephone voice transmission applications. The criticality of the conductor offset dimension can perhaps best be appreciated by the fact that in one particular type of flat cable, where the nominal offset dimension is 0.010 inch, even several lateral conductor deviations greater than .+-.0.002 inch in a given length of cable as short as 10 feet, can produce a drastic decrease in crosstalk loss from a range from 110-120 db down to an unacceptable level well under 100 db.
In an effort to reduce the conductor spacing deviations in question, a number of different types of conventional flat cable laminating apparatus, and process techniques, have been tried, but with no success. In this regard, different combinations of laminating roller assemblies, in particular, were tried wherein the rollers, for example, comprised either two steel or one steel and one rubber-covered roller, either selectively or both internally heated, and heated in different ways, for laminating two offset arrays of ribbon-shaped conductors between three polyester plastic films, wherein the two outer films were adhesive-backed on the inner side, and the center film was adhesive-backed on both sides. Such a combination of adhesive-backed films was considered important so as to achieve a reliable and high degree of bond strength, and very precisely positioned, permanently bonded and completely encapsulated conductors between the center and associated outer films. Concomitantly, the laminating temperature, pressure and feed rate employed were also varied over wide ranges, selectively, in attempts to maintain conductor spacing uniformity within the necessary stringent limits.
Unfortunately, however, it was found that no prior conventional laminating apparatus, nor any particular choice of laminating operating parameters employed in conjunction therewith, would consistently maintain the requisite conductor spacings and offset within an acceptable range of .+-. deviations and, particularly, while at the same time producing satisfactory bond strength between the three films of the composite cable. Indeed, not even a change in the type of plastic films employed, or changes in the thicknesses thereof, or in the composition or thicknesses of the adhesive coatings, proved beneficial in maintaining either the conductor spacing or offset within the required tolerances.
In addition to the aforementioned problems experienced heretofore in manufacturing a cable of the type of concern herein, another problem also arose with respect to terminating such a cable for connectorization. More specifically, because of the utilization of two arrays of conductors, with each laminated between an outer and center film, both adhesive-backed, the interposed conductors were firmly bonded to both films. As such, whenever short terminating segments of the outer films were removed, such as through the use of a conventional material grinding or abraiding operation, so as to expose the outer surfaces of the terminated conductors, they still remained firmly secured to the center film. This, of course, made complete insulation stripping of the terminating conductor ends quite difficult and time consuming.
For similar reasons, the prior use of conductor masking or separating tapes, disposed transversely across, and on opposite sides of, a single array of conductors, so as to facilitate the separation of two opposite side cover films from the conductors, likewise would not prove effective in terminating the flat cable in question for connectorization. More specifically, in accordance with the prior disclosed uses of such tapes, they would not be effective in separating two underlying/overlying arrays of conductors from a normally inaccessible center film (as distinguished from cover films) bonded to both arrays of conductors.
W. B. Fritz et al. U.S. Pat. No. 4,149,026 discloses another technique employed heretofore to separate the terminating ends of two underlying/overlying arrays of conductors when interleaved and laminated between two respectively associated adhesive backed outer carrier members and an inner adhesive member. As disclosed in that reference, the inner adhesive member is dimensioned such that it does not extend to both marginal edges of the outer carrier members. One longitudinally disposed edge of each carrier member is thus readily accessible for peeling the conductors bonded thereto from the inner member, provided the conductors are initially bonded more firmly to the carrier members than to the inner adhesive member. Disadvantageously, such a cable construction does not allow the two arrays of conductors to be completely hermetically sealed between the outer carrier members, as is desired, if not required, in many flat cable wiring applications.
In view of the foregoing, there has been an urgent need for an improved type of underlying/overlying multiconductor flat cable, and of a method and apparatus for the manufacture thereof, that will allow adjacent conductor pairs therein to be consistently spaced within extremely stringent limits, while positioned in the aforementioned, and advantageous, parallelogram type of geometric configuration, desired for high crosstalk rejection. A solution has also been sought for such an improved cable which at the same time will allow for the simplified termination and connectorization thereof.